The present invention is directed to a visual signaling system and, more specifically, a strobe circuit for a visual signaling system.
Signaling systems have been utilized in various areas of commercial and residential buildings, as well as in outside areas, for the purpose of attracting the attention of a person to, for example, warn of a danger of a fire or to alert such persons to a telephone call, damong other purposes. Many of these visual signaling systems include one or more visual signaling devices that have a flash tube that contains a gas, e.g., Xenon, that is ionized at the direction of a control circuit that receives power from a power supply, typically located at a control panel. Visual signaling systems have generally included a number of visual signaling devices coupled in parallel to the power supply, which supplies power to the devices. The visual signaling devices typically include one or more storage capacitors for storing the energy used to flash the strobe. In such visual signaling systems, the current drawn by the visual signaling devices may exceed the capability of a power supply (e.g., a DC power supply), upon initial power-up, due in large part to the discharged state of the one or more filter and storage capacitors.
As such, a number of techniques have been implemented to limit an in-rush current to a particular visual signaling device. For example, one device implements a thermistor between filter and storage capacitors and a power supply. The thermistor functions to initially limit an in-rush current at power-up and, as it continues to heat, transitions to a lower resistance such that the resistance is substantially reduced between the power supply and the storage capacitor of the visual signaling device.
Another signaling device uses a bipolar pass transistor whose emitter and collector are in series between a power supply and a storage capacitor. The device includes a series resistor coupled between the emitter of the pass transistor and the power supply. A base of a bipolar shut-off transistor is coupled to the emitter of the pass transistor. A collector of the shut-off transistor is coupled to a base of the pass transistor and an emitter of the shut-off transistor is coupled to the power supply. When an in-rush current threshold value (set by the value of the series resistor) is exceeded, the shut-off transistor turns on thereby limiting the base current of the pass transistor, which limits the current flowing into the storage capacitor through the pass transistor.
Yet another device uses a field effect transistor (FET) whose source is coupled to a power supply and whose drain is coupled to a storage capacitor. A resistor, coupled across the source and drain of the FET, is utilized to limit in-rush current. A control circuit, coupled to a gate of the FET, turns the FET on to provide a low impedance path between the power supply and the storage circuit after an elapsed time interval. While the prior art signaling devices function, they have not, in general, charged the flash circuit in an efficient manner, maintained a controlled breakover voltage for the flash tube and provided selectable flash intensities.
Thus, it would be desirable to provide a visual alarm device including a strobe circuit that reduces in-rush current while efficiently charging a storage circuit, provides selectable flash intensities and maintains a controlled breakover voltage for a flash tube.
An embodiment of the present invention is directed to a visual alarm device that includes a flash tube and a strobe circuit. The flash tube provides a visual indication responsive to a trigger signal. The strobe circuit is coupled between the flash tube and a power source and causes the flash tube to emit light at a desired flash rate. In one embodiment, the strobe circuit includes a storage circuit, an in-rush current limiting circuit and a control unit. The in-rush current limiting circuit is coupled between the storage circuit and the power source and includes a first switching device with a control input. The control unit is coupled to the control input and periodically asserts a control signal on the control input, which causes the first switching device to provide power to the storage circuit. The control signal includes a plurality of fixed duration pulses provided during a first predetermined time period.